Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!timbuk!cs.umn.edu!ux.acs!vx.acs.umn.edu!dhoyt From: dhoyt@vx.acs.umn.edu (DAVID HOYT) Newsgroups: comp.sys.mac.hardware Subject: Re: Memory speeds can be critical (was: SIMMs for IIsi - what do I need?) Message-ID: <2924@ux.acs.umn.edu> Date: 19 Dec 90 18:47:35 GMT References: <110992@convex.convex.com> <2915@ux.acs.umn.edu> Sender: news@ux.acs.umn.edu Reply-To: dhoyt@vx.acs.umn.edu Organization: University of Minnesota, Academic Computing Services Lines: 30 News-Software: VAX/VMS VNEWS 1.3-4 In article , amanda@visix.com (Amanda Walker) writes... >Nope. Sigh. I have no idea what you're talking about, but it sure >isn't DRAM. Well, yes and no. I realize that there is no 'signal' that goes through the chip. I used that 'simplification' as a way of explaining what is really going on, but that is not really goes on. Think of a straight wire with zero impedance. A signal will travel through it at a rate of about 30cm/ns. Boost the impedance on the wire and the signal take longer to travel the same distance. The same thing happens with memory chips. The lower speed ones tend to have higher impedances. The higher speed chip will have it's downbeat a bit before the slower chip. In micros there are fairly few components and the memory drivers are fairly simple; so using different speed chips usually are not a problem as long as the speeds are fairly close. In a more complex system, such as a mainframe, all of the small differences in speed could add up to a big difference in the end. With the end result of the computer not working. This is why mainframe computer engineers specify both the best and the worst behavior allowable to their chip suppliers. With micros, because of the reduced complexity, you can get away with a little bit more slack at either end. But you can still run into problems. If you put 4ns memory in your mac most likely it wouldn't work. The advice of Apple is sound. Don't mix if you don't have to mix. And if you do mix, don't mix in the same bank. david | dhoyt@vx.acs.umn.edu | dhoyt@umnacvx.bitnet Worry about it a little bit, but that's all. -- John Hartford